A grating made in this way is said to be of varying pitch, while a grating whose pitch remains the same over its entire length is said to be of fixed pitch. A grating in which the optical phase shift is identical over the pitch of the entire grating is said to be a grating of fixed phase shift. In the context of optical fiber information transmission systems, a varying pitch may be desirable so as to give the grating either a broader spectrum band, or else to achieve spectrum dispersion that is adapted to a particular requirement such as compensating for the chromatic dispersion of a line fiber.
In the present invention, the grating may be induced in the fiber using an initial pitch that is fixed, with said pitch subsequently being modified locally so as to obtain a final pitch having the desired variation. Pitch modification over time may also be useful in correcting interfering variations in pitch that can arise along a grating while the grating is being induced to the fiber, e.g. when it is desired to make a grating of fixed phase shift.
The grating is typically induced by phase shifting of a photorefractive effect that is achieved by interference between two ultraviolet light beams. Interfering variations in pitch can be the result, for example, of Gaussian distribution of light intensity in the inducing beams.
Various ways in which a grating can be induced by the photorefractive effect are described in an article entitled "Photoinduced Bragg gratings in optical fibers" by W. Morey, G. A. Ball, G. Meltz, published in Optics & Photonics News, February 1994, pp. 8-14. Another technique using a mirror is described in an article entitled "Realisation d'un filtre de Bragg photoinscrit dans une fibre optique a l'aide d'un montage interferometrique constitue d'un miroir unique"["Making a photoinduced Bragg filter in an optical fiber by means of an interferometer setup using a single mirror"], by I. Riant, P. Sansonetti, H. Fevrier, S. Artigaud, 13emes Journees d'Optique Guidee; 1993.
A first method is known for modifying the pitch of the grating induced in an optical fiber over the course of time. That method consists in subjecting the fiber to traction, thereby lengthening the fiber elastically and thus increasing the pitch of the grating. All the gaps between successive diffraction elements are then increased by the same amount so that if the initial pitch was constant, then the final pitch as obtained by that method is also constant.
A second known method is used to inscribe a varying pitch grating directly in a fiber by the photorefractive effect. Grating pitch variation is the result of variation in the width of interference fringes along the length of the waveguide. That second method is described in "Fiber dispersion compensation using a chirped in-fiber Bragg grating" by J. Williams, I. Barnian, K. Syndent, and N. J. Drau, Elect. Lett., 1994, Vol. 30, No. 12, pp. 985-987.
A third known method has been proposed for achieving the same object by means of a system using interference fringes of constant width. An optical waveguide in which a grating is to be induced is carried on a curved line of a support so that the inclination of light rays relative to the axis of the waveguide varies along the length of the waveguide. That third known method is described in patent document WO-A-86 01303 (United Teleologies). It is also described in the context of an optical filter in the article entitled "Chirped gratings produced in photosensitive optical fibers by fiber deformation during exposure", Elect. Lett., 1994, Vol. 30, No. 5, pp. 440-442.
A fourth known method is described in the document U.S. Pat. No. 4,474,427 (Hill et al.). That document teaches firstly how to achieve time variation of pitch in a grating induced in an optical fiber. That variation is obtained by subjecting the fiber to mechanical tension. The document also teaches how to achieve longitudinal pitch variation by causing the tension to vary longitudinally: for that purpose the fiber is wound on a conical mandrel.
A fifth known method is described in the document "Strain gradient chirp of fiber Bragg gratings" by P. C. Hill, published in Electronics Letters, Vol. 30, No. 14, Jul. 7, 1994, pp. 1172-1174. To achieve longitudinally varying tension in an optical fiber, a tension force is applied to one end of fiber and it is taken up in a longitudinally progressive manner by an adhesive that is subject to creep under tension.